(1) Field of the Invention
The present invention relates to a thermal head to be used for heat-sensitive recording. More particularly, the present invention relates to a thermal head excellent in both the temperature rising and falling characteristics at the thermal recording.
(2) Description of the Prior Art
A conventional thermal head is formed, for example, by laminating on the surface of a glaze layer 12 on a substrate 11 heat-generating resistors 13a composed of tantalum nitride (Ta.sub.2 N), pairs of electrodes composed of aluminum or the like (only one electrode 13b is shown) and a protecting layer 14 composed of tantalum pentoxide (Ta.sub.2 O.sub.5), as shown in FIG. 2. A certain voltage is applied to the heat-generating resistors 13a through the electrodes 13b to generate Joule heat selectively in the heat-generating resistors 13a to exert the function of the thermal head. The substrate 11 is formed of a ceramic such as alumina, and in order to improve the thermal response characteristics, the glaze layer 12 is formed of a glass composed mainly of silica (SiO.sub.2) or the like. A heat-generating element 13 is constructed by the heat-generating resistor 13a and electrode 13b.
In this conventional thermal head, many heat-generating elements 13 are formed very adjacently to one another on the glaze layer 12, and Joule heat generated in one heat-generating element 13 has influences on the adjacent heat-generating element. Accordingly, in the case where the heat-generating element 13 is selectively actuated to generate heat, the printing density is changed according to whether or not heat is generated in the adjacent heat-generating element.
Furthermore, in the conventional thermal head, when one heat-generating element 13 is actuated to generate heat, the glaze layer 12 is heated not only in the portion just below this heat-generating element 13 but also in the surrounding portion. Accordingly, the heat capacity of the glaze layer 12 is much increased and the rising response characteristics for elevating the temperature of the heat-generating element 13 to a desired level necessary for printing are degraded.
As means for eliminating this defect of the conventional thermal head, there has been proposed a thermal head in which the glaze layer 12 is divided into parts for the respective heat-generating elements 13 (see, for example, Japanese Utility Model Application Laid-Open Specification No. 102051/81).
However, in this thermal heat having the glaze layer 12 divided into parts for the respective heat-generating elements 13, heat generated by one heat-generating element 13 is absorbed only in the glaze layer 12 just below this heat-generating element 13, and the quantity of absorbed heat is small and the rising response characteristics of the heat-generating elements 13 are improved. However, since the contact area between the glaze layer 12 and the substrate 11 is very narrow, conduction of heat absorbed in the glaze layer 12 to the substrate 11 is poor and a long time is required for reduction of the temperature. Accordingly, the falling response characteristics of the heat-generating elements 13 are bad, and the printing density obtained in case of continuous printing is greatly different from the printing density obtained when printing is carried out at intervals.